This invention relates generally to magnetic recording channels and, more particularly, to magnetic recording channels with improved sync mark design and detection using correlation.
Data is stored on contemporary magnetic hard disk drives in concentric tracks around the recording surface or surfaces of the disk or disks. Each track may be divided into a number of addressable sectors, with each sector including a preamble, sync mark, and user data.
The preamble may, for example, contain a portion that enables the read channel to adjust its gain and allow a phase locked loop (PLL) to achieve bit synchronization. The preamble may also include a DC erase portion in which there are no logical transitions (e.g., an uninterrupted string of zeros) for a specified length. Since a string of bits with no logical transitions may be illegal everywhere else on the disk, the DC erase portion of the preamble may uniquely identify this portion as being part of the sector preamble.
The sync mark may follow the preamble on the disk and may be designed to be of any suitable length. Since the next bit of data after the sync mark starts the user data, it is crucial that the sync mark be easily detected and detected extremely reliably. Current sync mark designs may impose several restraints on the sync mark to increase detection probability, including, for example, no T-spaced transitions and no transitions on byte boundaries, to reduce or eliminate most-likely error events.
As data densities and data rates continue to increase, however, more noise is inevitably introduced into magnetic data channels. For example, in order to increase the areal densities of magnetic media, many media manufacturers are using perpendicular recording. Unlike traditional longitudinal recording, where the magnetization is lying in the plane of the magnetic medium, with perpendicular recording the media grains are oriented in the depth of the magnetic medium with their magnetization pointing either up or down, perpendicular to the plane of the disk. Using perpendicular recording, manufacturers have exceeded magnetic recording densities of 100 Gbits per square inch, and densities of 1 Terabit per square inch are feasible.
As storage densities increase, the signal processing and detection of magnetic channels becomes more difficult. Sources of distortion, including media noise, electronics and head noise, inter-track interference, thermal asperity, partial erasure, and dropouts, are becoming more pronounced. Particularly troublesome are signal-dependent types of noise, such as transition jitter, because these types of noise are quickly becoming the dominant sources of detection errors.
Because bit error rates (BER) in these new magnetic recording channels are increasing, a new sync mark design and detection scheme is needed to more reliably detect disk sync marks. Traditional sync mark detection schemes using Viterbi detection may be inadequate to support these high data rate channels. If a disk sync mark is not reliably detected on the first attempt, the disk spindle motor typically must drive the disk completely around again and attempt redetection of the sync mark. This reduces drive data rates and cripples overall system performance.
Accordingly, it is desirable to provide systems and methods for enhanced sync mark detection using correlation. The enhanced sync mark design and detection scheme may provide a gain of several dB over traditional sync mark designs.